Stability of quiescent operating point:

Let us assume that the transistor is replaced by an other transistor of same type. The b_dc of the two transistors of same type may not be same. Therefore, if b_dc increases then for same I_B, output characteristic shifts upward. If b_dc decreases, the output characteristic shifts downward. Since I_B is maintained constant, therefore the operating point shifts from Q to Q₁ as shown in fig. 5. The new operating point may be completely unsatisfactory.

Therefore, to maintain operating point stable, I_B should be allowed to change so as to maintain V_CE & I_C constant as b_dc changes.

Fig. 5

A second cause for bias instability is a variation in temperature. The reverse saturation current changes with temperature. Specifically, I_CO doubles for every 10^oC rise in temperature. The collector current I_C causes the collector junction temperature to rise, which in turn increases I_CO. As a result of this growth I_CO, I_C will increase ( b_dc I_B + (1+ b _dc ) I_CO ) and so on. It may be possible that this process goes on and the ratings of the transistors are exceeded. This increase in I_C changes the characteristic and hence the operating point.

Stability Factor:

The operating point can be made stable by keeping I_C and V_CE constant. There are two techniques to make Q point stable.

1. stabilization techniques
2. compensation techniques

In first, resistor biasing circuits are used which allow I_B to vary so as to keep I_C relatively constant with variations in b_dc , I_CO and V_BE.

In second, temperature sensitive devices such as diodes, transistors are used which provide compensating voltages and currents to maintain the operating point constant.

To compare different biasing circuits, stability factor S is defined as the rate of change of collector current with respect to the I_CO, keeping b_dc and V_CE constant

S = ¶ I_C / ¶ I_CO

If S is large, then circuit is thermally instable. S cannot be less than unity. The other stability factors are, ¶ I_C / ¶ b_dc and ¶ I_C / ¶ V_BE. The bias circuit, which provide stability with I_CO, also show stability even if b and V_BEchanges.

I_C = b _dcI_B + (I + b _dc ) I_CO

Differentiating with respect to I_C,

In fixed bias circuit, I_B & I_C are independent. Therefore and S = 1 + b_dc. If b _dc=100, S = 101, which means I_Cincreases 101 times as fast as I_CO. Such a large change definitely operate the transistor in saturation.

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